A known fuel injector 1, as illustrated in FIG. 1, comprises an injector body 60, an injector nozzle 62 and a movably mounted injector needle 64. A spring 66 is provided in a spring chamber 68 for biasing the injector needle 64 towards a seated position in which injection of fuel through one or more spray holes (not shown) provided in the injector nozzle 62 to a combustion chamber (not shown) is prevented.
The injector 1 further comprises a control valve assembly 2, which controls movement of the injector needle 64. The control valve assembly 2 comprises a control valve body 6, a control valve member 4 which is reciprocally movable within a bore 8 provided in the control valve body 6, and a control chamber 40 located at an upper end of the bore 8. An actuator comprising an armature 70 is provided to control movement of the control valve member 4 between an open and a closed position.
Leakage of fuel within the control valve assembly 2 is a known problem. In particular; leakage can occur between the control valve member 4 and a guide portion of the bore 8 of the control valve body 6.
Increasing fuel pressure within the bore 8 causes the guide portion of the bore 8 to expand, thereby increasing clearance between the control valve member 4 and the control valve body 6, resulting in an increased leakage path. Increased leakage has a detrimental effect on the efficiency of the injector 1, for example by increasing the ratio of quantity of leaked fuel against quantity of injected fuel.
One solution to the problem of leakage along the guide portion of the bore is to reduce distortion of the control valve member and the control chamber. In the prior art control valve assembly 2 embodiment illustrated in FIG. 2, this is achieved by providing a pressure compensating chamber 80, defined by an outer annular recess of a sleeve or insert 82 located within the control valve body 6. It is necessary to provide an aperture 84 within the insert 82 to allow the high pressure fuel supply line 52 to communicate with the control chamber 40. Furthermore, precise machining of an upper internal frustoconical face 86 the insert 82, is required, as this face defines a valve seat for the control valve. In addition, installation of the insert 82 must be high precision, to avoid any leakage at the guide portions of the insert 82, thereby minimising distortion of the insert 82, whilst still enabling sufficient deformity of the insert 82 to provide the required compensation against pressure.